I'd be inclined to get the 2" 4x powermate and use it with your existing 90, but I'm not sure that will work with your camera. There is also the t-adapter for it:
http://www.optcorp.com/te-ptr-4201-4...ter-2inch.html
Atmospherics tend to be a problem more than anything, I think the resolution you will get out of the 90mm will be very good anyway and it is a capable scope. I used a powermate with my 105mm triplet and was very happy with the results.
If I was going to buy a new scope though, I'd go for a 120mm rather than the 100, seeing that you already have the 90.
The other option of course is to use a camera with a smaller chip and capture video to stack. Something like the ASI cameras do a great job if you aren't wanting to print the end result.